The present invention relates to a sensor apparatus applied to a distance measuring apparatus and a method for controlling the sensor apparatus and, more specifically, a sensor apparatus using a ring-shaped charge transfer device, applied to an automatic focusing mechanism of a camera, and a method for controlling the sensor apparatus.
Conventionally, a distance measuring device which circulates accumulated charges, obtained by performing photoelectric conversion of light reflected by an object to be measured, using a ring-shaped charge transfer device, such as CCD, to integrate the light and measures the distance to the object on the basis of voltages of the integrated charges has been proposed by the Japanese Patent Publication No. 5-22843. Further, a modified distance measuring apparatus, which is briefly shown in FIG. 5, has been proposed by the Japanese Patent Application Laid-Open No. 8-233571.
Referring to FIG. 5, a light-emitting device 1 turns on and off at predetermined period. Light emitted by the light-emitting device 1 projects on an object (not shown) via a projection lens 2. From the object, light due to the light emitted by the light-emitting device 1 when it is on and external light, and light due to external light when the light-emitting device 1 is off is alternately received. The reflected light incidents on a sensor array 55 composed of sensor elements, i.e., photoelectric conversion elements, S1 to S4 of a sensor apparatus 54 via a light-receiving lens 3.
Charges generated by the sensor elements S1 to S4 while the light-emitting device 1 is on are transferred to accumulation units ST2 via an electronic shutter ICG and sorting units ST01 to ST04. Similarly, charges generated by the sensor elements S1 to S4 while the light-emitting device 1 is off are transferred to accumulation units ST1 via the electronic shutter ICG and the sorting units ST01 to ST04.
Note, an initialization unit CCLR performs initialization before initiating integration of charges. During the initialization, the sorting units ST01 to ST04 are not operated so that no charge is transferred to the accumulation units ST1 and ST2. Further, the electronic shutter ICG is for controlling an amount of charge to be accumulated when a charge converted from light is very large. The electronic shutter ICG also has a function of draining charges generated by the sensor elements S1 to S4 during the initialization of pixels C1 to C8 of a ring-shaped transfer unit 7, which will be explained later.
After charges generated while the light-emitting device 1 is on and charges generated while the light-emitting device is off are accumulated in the accumulation units ST2 and ST1, respectively, they are transferred to pixels A1 to A8 of a charge transfer unit 56 via shift gates SH, further transferred to the pixels C1 to C8 of the ring-shaped transfer unit 7 via the pixels B1 to B4 in synchronization with the on/off operation of the light-emitting device 1. Since the emission of light and the transference of charges are performed in synchronization with each other, charges converted by the respective sensor elements S1 to S4 when the light-emitting device 1 is on and off are respectively accumulated in the pixels C1 to C8.
A skim unit 8 composed of elements SK1 to SK3 skims a predetermined amount of charges which are unnecessary if a pair of charge amounts, obtained when the light-emitting device 1 is on and off, are large, via a clear unit SCLR, thereby preventing the pixels C1 to C8 of the ring-shaped transfer unit 7 from being saturated. Note, since charges are accumulated in the pairs of accumulation units ST1 and ST2, corresponding to the respective sensor elements S1 to S4, by obtaining a difference between the charges accumulated in each pair of the accumulation units ST1 and ST2, a signal component only due to light emitted by the light-emitting device 1 is obtained.
A distance measuring operation in a so-called active mode performed by emitting light is explained above. By keeping the light-emitting device 1 off, omitting skimming operation by the skim unit 8, and reading out the signals from the pixels C1 to C8 without taking differences between charges corresponding to pairs of the accumulation units ST1 and ST2, a known distance measuring operation is performed in a passive mode.
According to the configuration as described above, by turning on/off the light-emitting device 1 repeatedly, a signal component from which effects of external light and noise are removed is obtained as an output signal S from the ring-shaped transfer unit 7 via an amplifier FG. Thus, by using two sensor devices, having the same configuration as the sensor device 54, arranged at a predetermined interval, and performing correlation operation utilizing the principle of trigonometry on the basis of output signals S from the respective sensor devices, the distance to the object is measured with high precision.
The above explanation, with reference to FIG. 5, is about a light beam t2 emitted from the center of the light-emitting device 1, and reflected light r2 due to the light beam t2 is received by the sensor elements S1 to S4 of the sensor array 55. However, if light beams t1, t2 and t3 shown in FIG. 5 are projected onto an object or objects in order to measure distances to a plurality of points in a wide angle of view, reflected light r1 and r3 enter the lens 3 at large incident angles, and the length of the sensor array 55 may be too short to receive the reflected light r1 and r3.
In this case, the number of sensor elements may be increased to, e.g., twice as many as the number of the sensor elements S1 to S4; however, as the number of the sensor elements increases, the numbers of the sorting units, accumulation units, pixels of the charge transfer unit 56, and pixels of the ring-shaped transfer unit 7 must be increased, which makes the configuration complicated. Further, since one circulation of charges in the ring-shaped transfer unit 7 should synchronize with one on/off operation of the light-emitting device 1, the charging period is lengthened as the number of pixels of the ring-shaped transfer unit 7 is increased, and it takes a long time to complete one distance measuring operation. In addition, since the amount of charge which can be discharged in one circulation of the ring-shaped transfer unit 7 by the skim unit 8 is fixed, the ability of removing charges due to external light deteriorates, thus the precision of a distance measuring operation is impaired.
To overcome the above problem, an apparatus in which the number of pixels of the sensor array is increased for performing a multi-point distance measuring operation while maintaining the number of pixels of a ring-shaped transfer unit as that in the apparatuses as those shown in FIG. 5 is disclosed in the Japanese Patent Application Laid-Open No. 8-320223. In the apparatus, however, more transfer pixels of a linear transfer unit for transferring charges are necessary in addition to the transfer pixels A1 to A8, thus, the size of the chip becomes large. Furthermore, it is necessary to increase the driving frequency of the transfer pixels to a higher frequency, e.g., twice higher than the driving frequency of the ring-shaped transfer unit, which complicates control of the apparatus.